A car air-conditioner including an electric compressor (hereinafter referred to simply as a compressor) which employs a sensor-less dc brush-less motor as a driver, is disclosed in, e.g., Japanese Patented Publication No. JP06-156055. This air-conditioner is shown in FIG. 21 which illustrates a schematic structure of this conventional car air-conditioner including a compressor.
In FIG. 21, air duct 101 sucks air from air inlet 103 due to an operation of indoor fan 102, and blows the air through indoor heat exchanger 104 into the compartment of the vehicle through air outlet 105. Heat exchanger 104 disposed in air-duct 101 forms a refrigerating cycle together with compressor 106 driven by a sensor-less dc brush-less motor, four-way switching valve 107 for selecting heating or cooling by switching the flow of refrigerant, and outdoor heat exchanger 110 which exchanges heat of indoor air with that of outdoor air using the operation of throttle 108 and outdoor fan 109.
Air-conditioner controller 112 controls the operations of inverter 111 which operates the sensor-less dc brush-less motor (not shown), i.e., the driver of compressor 106, indoor fan 102, four-way valve 107 and outdoor fan 109.
Air-conditioner controller 112 is connected to various switches such as indoor-fan switch 113 for setting ON/OFF, strong/weak of indoor fan 102; air-conditioning switch 114 for selecting cooling, heating or OFF; and temperature-adjustment switch 115. Controller 112 is also connected to communicator 116 for communicating with a vehicle controller (not shown). In the foregoing structure, e.g., air-blow is started by switch 113, and a weak level of blow is selected, then cooling is instructed by switch 114. Controller 112 sets valve 107 as shown with a solid line in FIG. 21, and works indoor heat exchanger 104 as an evaporator and works outdoor heat exchanger 110 as a condenser, and turns on outdoor fan 109, then operates indoor fan 102 on a weak level.
A temperature of indoor heat exchanger 104 is adjusted in accordance with the setting done by switch 115 taking advantage of variable rpm of compressor 106, where the rpm is varied by inverter 111. When cooling/heating is turned off by switch 114, compressor 106 and outdoor fan 109 are also turned off.
Turning off fan switch 113 turns off indoor fan 102, then compressor 106 and outdoor fan 109 are also turned off for protecting the refrigerating cycle. On the other hand, an instruction of turning off the air-conditioner is given from a vehicle controller (not shown) via communicator 116 in order to save power or protect the battery, wherein controller 112 handles this instruction in the same manner as turning off the air-conditioner by switch 114.
When inverter 111 is powered at 120-degree interval drive, the magnetic field changes at 60-degree intervals (power is fed at 60-degree intervals), so that the sensor-less dc brush-less motor (not shown), which drives compressor 106, tends to produce fluctuations in torque.
A circuit of this power feeding at 120-degree intervals is shown in FIG. 22, where inverter 111 is coupled with battery 121 (power supply). The dc brush-less motor is operated by inverter switching-element 122 coupled to battery 121 and the controlling of inverter diode 123. The dc brush-less motor comprises stator winding 124 and magnet rotor 125, and is coupled to battery 121 via inverter 111.
Inverter 111 comprises the following elements:
current sensor 126 for detecting a current of the power supply, thereby calculating a power consumption and protecting the switching elements;
phase shift circuit 127 for detecting a position of magnet rotor 125 from a voltage of stator winding 124; and
phase comparison circuit 128.
Control circuit 129 controls ON/OFF of switching element 122 based on signals supplied from sensor 126 and circuit 128.
The car air-conditioner including the foregoing compressor is subject to a thermal load environment different from that of a room air-conditioner. Although a vehicle has a small compartment, it has a rather large area occupied by windows. Since a vehicle travels frequently through a sunny place and shade, it is subject to solar radiation, and the thermal load in the compartment fluctuates frequently. The start/stop of operating the compressor depends on switch 114, switch 113, or a temperature adjusting operation set by switch 115, so that the compressor mounted in a car is frequently started or stopped compared with that of a general room air-conditioner.
Since the compressor mounted in a car is frequently started or stopped, it is required to start or stop before a high pressure side and a low pressure side of the refrigerating cycle are balanced with each other. In other words, the compressor is started frequently while a large pressure difference still remains.
Therefore, the driver of the compressor mounted in a car needs performance considering every possible operating condition, in particular, the performance of starting the compressor even if a large pressure difference still remains. This performance is hereinafter referred to as start-performance under pressure difference.
To be more specific, the compressor using HFC134a refrigerant is required to start even if the difference between a discharge pressure and a suction pressure is as high as 2.0 MPa. This pressure difference is as much as several times that of a driver of a general room air-conditioner which does not need the start-performance under pressure difference. A method of boosting start-torque by increasing a voltage (duty ratio) at the start is proposed for a conventional driver of a compressor, e.g., disclosed in Japanese Patent Application Non-Examined Publication No. JP10-7255. In this case, a starting current also increases, so that a threshold value for current protection also increases.
However, the methods of increasing only a starting voltage, a duty ratio, or a current threshold value for improving the start-performance under pressure difference involves increasing the current in a large amount at the same time. Therefore, those methods can start the compressor under a certain level of pressure difference (according to the experience, the compressor can be started under the pressure difference up to 0.8 MPa). However, when the pressure difference in the refrigerating cycle exceeds the certain level, an over-current protection is activated, so that the compressor cannot start.
The rotor fails in following the rotating magnetic field due to a large torque load, so that the compressor falls in out of sync. at the start. Once the compressor falls in this state, a positional detection becomes unstable, which disables the compressor from starting.
If the compressor cannot start, the crew and passengers of the vehicle have to wait until the pressure difference in the refrigerating cycle falls within a range allowing the start. During the waiting time, the temperature of the compartment rises in the condition of cooling operation, so that the crew and passengers feel uncomfortable. The vehicle has a large area occupied by windows, so that it is subject to solar radiation and the thermal load frequently fluctuates in the compartment. As a result, the crew and passengers feel much more uncomfortable.